1. Field of the Invention
The present invention is related to a ball bearing for rotationally supporting various types of mechanical equipment such as gas turbines, specifically for supporting main shafts and the like rotating at high speeds. Moreover, the ball bearings to which the present invention is addressed, include all types of radial ball bearings such as deep groove type and angular type, as well as thrust ball bearings.
2. Description of the Related Art
In order to support various types of rotating members such as the main shaft of a gas turbine, a ball bearing 1 such as shown in FIGS. 1-2 is widely used. This ball bearing 1 comprises a first race ring or inner ring 3 having a first surface or outer peripheral surface formed with a first race surface or inner ring raceway 2 thereon, a second race ring or outer ring 5 having a second surface or inner peripheral surface formed with a second race surface or outer ring raceway 4 thereon, and a plurality of balls 6 provided so as to be freely rotatable between the inner ring raceway 2 and the outer ring raceway 4. The plurality of balls 6 are retained so as to be freely rotatable by an annular retainer or cage 7, with circumferentially adjacent balls 6 spaced apart thereby.
With such a ball bearing 1, the inner ring 3 is externally secured for example to the main shaft (not shown) of a gas turbine, while the outer ring 5 is internally fitted into a housing (not shown), thereby supporting the main shaft so as to be freely rotatable within the housing. There has heretofore been little consideration particularly with regards to the imbalance (dynamic imbalance) in the balls 6 of the ball bearing 1 of this construction. In other words, the manufactured balls 6 are assembled between the inner ring raceway 2 and the outer ring raceway 4 without any special measurements being made for imbalance.
When the ball bearing 1 is used to support a rotating member rotating at a high rotational speed, imbalance of the balls 6 cannot be disregarded since the presence of any imbalance can cause damage to the balls. That is to say, according to research carried out by the present inventors, in the case of a ball bearing 1 used under conditions wherein the product d.sub.m .multidot.N of the pitch circle diameter d.sub.m mm of the balls 6 and the rotational speed N rpm of the inner ring 2 relative to the outer ring 5 is greater than 1,000,000 mm.multidot.rpm, then there is the likelihood of damage due to this imbalance.
The reason that such a ball bearing 1 incorporating balls 6 having an imbalance, is susceptible to damage at high rotational speeds, is considered to be due to the following.
When the balls 6 rotate at high speed during high speed use of the ball bearing 1, if there is a dynamic imbalance .DELTA.I in the balls 6, the locus of the rotation axis of the balls 6 becomes fixed. Specifically, although the balls are rotating, their rotation axis is always the same. Consequently, the rolling face of the balls 6 always contacts the inner ring raceway 2 and the outer ring raceway 4 over the same narrow strip portion. As a result, this contact area in the strip portion reaches its fatigue life at an early stage, becoming susceptible to damage. Moreover, if the balls 6 also have static imbalance, then since there will always be dynamic imbalance caused by the static imbalance, the rotational axis of the balls 6 will become fixed, with the likelihood of damage for the same reasons as above.